1. Field of the Invention
The invention relates generally to a boost circuit, and more particularly to, a boost circuit capable of constantly maintaining a pumping voltage regardless of variation in a power supply voltage.
2. Description of the Prior Art
In order to improve the level of integration and lower power consumption, a research on circuits that operates at a low operating voltage has recently been made actively. In order for the device to operate, a voltage higher than the power supply voltage (Vcc) is required as the operating voltage, if necessary. To this end, it is required that the power supply voltage be boosted to a target voltage. A boost circuit is a circuit for boosting the power supply voltage to the target voltage.
A construction of a conventional boost circuit will be described by reference to FIG. 1. FIG. 1 is a circuit diagram of the conventional boost circuit for explaining the construction and operation of the boost circuit.
The boost circuit includes a pumping capacitor C101, a precharge unit 110, a voltage dividing unit 120 and a kick signal generating unit 130.
The pumping capacitor C101 is connected between a first node N101 being an output node and a second node N102. The precharge unit 110 serves to apply the precharge voltage of the pumping capacitor C101 to the first and second nodes N101 and N102 according to a non-inverted pumping signal (BOOST). The voltage dividing unit 120 is connected between the second node N102 and a ground voltage (Vss) terminal and generates a divided voltage (VBREF). The kick signal generating unit 130 compares the divided voltage (VBREF) from the voltage dividing unit 120 with the reference voltage (VREF) and also applies the kick signal (VKICK) to the second node N102 according to the inverted pumping signal (BOOSTB) to boost the voltage of the first node N101 to the pumping voltage (VBOOST). A load capacitor C102 is connected between the first node N101 being the output node and the ground voltage (Vss) terminal.
In the above, the precharge unit 110 is connected between the power supply voltage (Vcc) terminal and the first node N101. The precharge unit 110 includes a first switching means P101 driven by the inverted pumping signal (BOOST), an inverting means L101 for inverting the non-inverted pumping signal (BOOST), and a second switching means N101 connected between the second node N102 and the ground voltage (Vss) terminal and driven by the output signal of the inverting means L101. At this time, the first switching means P101 may be implemented using a PMOS transistor, the second switching means N101 may be implemented using a NMOS transistor and the inverting means L101 may be implemented using an inverter.
Further, the voltage dividing unit 120 includes a plurality of resistors (only two first and second resistors are shown in the drawing, R121 and R122) serially connected between the second node N102 and the ground voltage (Vss) terminal. The plurality of the resistor R121 and R122 divide the voltage of the second node N102 to generate the divided voltage (VBREF).
Meanwhile, the kick signal generating unit 130 includes a switching means P131 connected between the power supply voltage (Vcc) terminal and the second node N102, for switching the power supply voltage (Vcc), a comparator 131 for comparing the divided voltage (VBREF) and the reference voltage (VREF), and a driving unit 132 to which the inverted pumping signal (BOOSTB) is applied as an enable signal, for driving the switching means P131 so that the kick signal (VKICK) is applied to the second node N102 according to the output signal of the comparator 131. At this time, the switching means P131 may be implemented using a PMOS transistor. The driving unit 132 includes a NOR gate device L131 having two inputted to which the inverted pumping signal BOOSTB) and the output signal of the comparator 131 are inputted, respectively, and an inverter L132 for inverting the output signal of the NOR gate device L131 to generate the driving signal (KICKB) of the switching means P131.
FIG. 2 is a graph illustrating the signal applied to the boost circuit in FIG. 1 and a waveform of a specific node. The operation of the conventional boost circuit will be described by reference to FIG. 1 and FIG. 2.
At a precharge period (A) in which the pumping signal (BOOST) is applied as a LOW level, the first switching means P101 is turned on by the pumping signal (BOOST) and the second switching means N101 is simultaneously turned on by the inverted pumping signal (BOOSTB) through the inverting means L101. Also, the inverted pumping signal (BOOSTB) is applied to the kick signal generating unit 130 to disable the kick signal generating unit 130. In more detail, the inverted pumping signal (BOOSTB) is also applied to the NOR gate device L131 included in the driving unit 131 of the kick signal generating unit 130. If the inverted pumping signal (BOOSTB) is applied, the NOR gate device L131 of the driving unit 131 generates a signal of a LOW level. This signal is then inverted by the inverting means L132. As the switching means P131 is turned of by the output signal (KICKB) of the inverting means L132, the power supply voltage (Vcc) is not applied to the second node N102. Therefore, the power supply voltage (Vcc) is transferred to the first node N101 and the ground voltage (Vss) is also transferred to the second node N102, through the first switching means P101 and the second switching means N101 that are turned on, so that the pumping capacitor C101 is precharged. At this time, the voltage dividing unit 120 outputs the divided voltage (VBREF) as the ground voltage (Vss) by the ground voltage (Vss) transferred to the second node N102.
Next, in a pumping period (B) in which the pumping signal (BOOST) is applied as a HIGH level, the first switching means P101 is turned off by the pumping signal (BOOST). The second switching means N101 is simultaneously turned off by the inverted pumping signal (BOOSTB) through the inverting means L101.
Meanwhile, the comparator 131 of the kick signal generating unit 130 compares the divided voltage (VBREF) being the ground voltage (Vss) and the reference voltage (VREF) to generate a signal of a LOW level. This signal is then is applied to the NOR gate device L131 along with the inverted pumping signal (BOOSTB) of the LOW level, so that the NOR gate device L131 generates a signal of a HIGH level. Next, the signal of the HIGH level of the NOR gate device L131 is applied to the switching means P131 as the signal (KICKB) inverted by the inverting means L132, so that the switching means P131 is turned on. The power supply voltage (Vcc) is applied to the second node N102 through the switching means P131 that is turned on, and the voltage of the first node N101 is simultaneously boosted to the pumping voltage (VBOOST) by the pumping capacitor C101 of the precharge state. At this time, in an initial period (Bb) of the pumping period (B), a little distortion occurs in the kick signal (VKICK) as the comparator 131 compares the divided voltage (VBREF) and the reference voltage (VREF). However, this distortion is stabilized soon.
Through the above operation, the pumping voltage (VBOOST) higher than the power supply voltage (Vcc) is generated. The pumping voltage (VBOOST) is supplied to a device requiring a high voltage as the operating voltage.
In the above boost circuit, the power supply voltage (Vcc) is used as the precharge voltage of a positive voltage. If the power supply voltage (Vcc) is increased, the amount of the precharge voltage is accordingly increased. Thus, the pumping voltage (VBOOST) is generated as a voltage higher than a target voltage. As such, if the pumping voltage (VBOOST) is generated as the voltage higher than the target voltage, the device is overworked and the device may be damaged.
Therefore, in order to prevent this, the reference voltage (VREF) is lowered if the power supply voltage (Vcc) is supplied high, so that the pumping voltage (VBOOST) is constantly generated as the target voltage.
This method, however, may be applied only when the capacity of the pumping capacitor C101 is higher than the capacity of the load capacitor C102. If the capacity of the pumping capacitor C101 is lower than that of the load capacitor C102, the pumping voltage (VBOOST) is lowered as the capacity of the load capacitor C102 is increased. Thus, there is a problem that the operating voltage is not sufficiently applied to the device.
The present invention is contrived to solve the above problems and an object of the present invention is to provide a boost circuit capable of improving the operating characteristic and reliability of the circuit in such a manner that a precharge voltage of a positive voltage applied to a capacitor is supplied as a constant voltage regardless of a power supply voltage and a pumping voltage is thus stably and constantly generated.
In order to accomplish the above object, the boost circuit according to the present invention, is characterized in that it comprises a pumping capacitor connected between first and second nodes, a precharge unit for applying a precharge voltage of the pumping capacitor to the first and second nodes according to an inverted pumping signal, a voltage dividing unit connected between the first node and a ground voltage terminal, a kick signal generating unit for comparing a divided voltage generated from the voltage dividing unit and a reference voltage and simultaneously generating a kick signal to the second node according to a non-inverted pumping signal, in order to constantly boost a voltage of the first node, and a boost unit using a voltage of the first node as the precharge voltage of a positive voltage, for boosting the power supply voltage up to a target voltage according to the non-inverted pumping signal.
In the above, the precharge unit comprises a first switching means connected between a power supply voltage terminal and the first node and driven by the inverted pumping signal, an inverting means for inverting the inverted pumping signal, and a second switching means connected between the second node and the ground voltage terminal and driven by the output signal of the inverting means.
The voltage dividing unit consists of a plurality of capacitors serially connected between the first node and the ground voltage terminal. The voltage of the first node is divided by the plurality of the capacitors so that the divided voltage is generated
The kick signal generating unit comprises a switching means connected between the power supply voltage terminal and the second node, for switching the power supply voltage, a comparator for comparing the divided voltage and the reference voltage, and a driving unit having an input terminal to which the non-inverted pumping signal is applied as an enable signal, wherein the driving unit drives the switching means according to the output signal of the comparator so that the kick signal is applied the second node. At this time, the driving unit comprises a NOR gate device having two input terminals to which the non-inverted pumping signal and the output signal of the comparator are inputted, respectively, and an inverter for inverting the output signal of the NOR gate device to generate a driving signal of the switching means.